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Building an ADU in Boston

City code and Massachusetts law · Last reviewed October 9, 2026

In Boston you can add one ADU inside your home if you own and live in a 1-, 2- or 3-family house. Outside Mattapan, an addition or a backyard ADU may need a variance from the Zoning Board of Appeal. The 2024 state ADU law does not apply in Boston, so the owner-occupancy rule still applies here. Every ADU needs a long-form building permit from Inspectional Services. Income-eligible owners can get a city grant of up to $7,500 and a 0% deferred loan of up to $50,000. [1][2][3][4]

Boston at a glance
Who can buildOwner-occupants of 1-, 2- or 3-family homes; condominiums and LLCs are not eligible [1]
Internal ADU (basement, attic)Allowed citywide in owner-occupied 1-3 family homes, using the existing footprint only [1][2]
Attached or detached ADUNot in the zoning code outside Mattapan; may need a Zoning Board of Appeal variance [1][2]
Mattapan detached ADU sizeFloor plate up to 900 sq ft or the main home's floor plate, whichever is less; up to 1.5 stories [6]
Mattapan addition ADU sizeUp to 75% of the main unit's square footage or 1,250 sq ft, whichever is smaller [6]
State ADU law (Affordable Homes Act)Does not apply in Boston [2]
Building permitLong-form permit; $50 plus $10 per $1,000 of estimated cost [3][8]
Zoning appeal$150 fee; file within 45 days of the refusal letter [3]
Permit review timeAbout 5 weeks if complete; longer with zoning relief [2][3]
City financial helpGrant up to $7,500 for design and permit costs; 0% deferred loan up to $50,000 for construction [4]
On this page
  1. At a glance
  2. What you can build in Boston
  3. Why the state ADU law doesn't apply here
  4. Mattapan: attached and detached ADUs by right
  5. Zoning changes in progress
  6. Permits, fees and timeline
  7. If you need zoning relief
  8. Grants and loans from the Boston Home Center
  9. Renting out your ADU
  10. Permit steps
  11. Who to call
  12. Questions people ask

What you can build in Boston

To build an ADU in Boston you must be an owner-occupant of a 1-, 2- or 3-family home in the city. Condominiums and LLCs are not eligible. If your home is in the FEMA flood zone, the city says to plan the ADU above the base flood elevation.

Boston recognizes three types:

Zoning allows every owner-occupied 1-, 2- and 3-family home in Boston to add one internal ADU. It must use the existing building footprint, with no new construction that adds square footage.

For attached and detached ADUs, the city says Boston's zoning code "does not currently include regulations" that allow them, except in part of Mattapan. Elsewhere you may need a variance from the Zoning Board of Appeal (ZBA).

Check your lot's zoning with the city's Zoning Viewer. [1][2]

Why the state ADU law doesn't apply here

The 2024 Affordable Homes Act made ADUs up to 900 sq ft by right in single-family districts across Massachusetts and banned owner-occupancy requirements. It did this by amending the state Zoning Act, Chapter 40A.

Boston says it gets its zoning power from a separate 1956 law, not Chapter 40A, so those changes "do not apply to Boston." The state's ADU regulation, 760 CMR 71.00, likewise covers only a city or town "subject to the provisions of M.G.L. c. 40A." In Boston, the city's own zoning rules described on this page still govern ADUs. Read our Massachusetts ADU guide for how the state law works everywhere else. [2][5]

Mattapan: attached and detached ADUs by right

Mattapan's residential zoning was updated in January 2024 to allow all types of ADUs without special approval. It applies only to parcels in both the Greater Mattapan Neighborhood zoning district and the PLAN: Mattapan study area, in the R1 or R2 subdistricts. Owner-occupants of 1-, 2- or 3-family homes there may add up to one non-detached ADU and one detached ADU.

Under the city's fact sheet for the Article 60 amendment:

Mattapan ADUs must also meet the height and size rules in Article 2 and the dimensional rules in Article 60, Table D. If your design doesn't comply, it may still need zoning relief. [1][3][6]

Zoning changes in progress

The city's Planning Department is working on a Neighborhood Housing zoning initiative. It would make it easier to build a detached ADU in a rear yard, among other changes. It is starting in Hyde Park, Roslindale and West Roxbury, with later phases for other neighborhoods.

As of October 2026 it is still a draft. Check the Neighborhood Housing page for its status before you assume it applies to your lot. [7]

Permits, fees and timeline

All ADUs need a long-form building permit from the Inspectional Services Department (ISD). The city recommends that your licensed contractor apply for all permits for you. You may also need electrical, mechanical, foundation and sprinkler permits.

Your plans must be stamped by a licensed architect or engineer, and you need a certified plot plan showing property lines. The long-form permit fee is $50 plus $10 for every $1,000 of estimated work cost, paid up front. A $100,000 project, for example, would pay $1,050.

The city says review may take up to 5 weeks if your application is complete. It takes longer if you need zoning relief or documents are missing. You can track it with the permit finder. The permit is valid for six months after it is issued, and you must work without interruption.

The city's rough timeline for a whole project is up to 3 months for design, up to 3 months for permitting and 6 to 12 months for construction. Its rough cost benchmark is $75,000 to $100,000 for an internal ADU and $250,000 to $350,000 for a detached one. See our ADU permit process guide and ADU cost guide. [2][3][8]

If you need zoning relief

During permit review, an ISD plans examiner checks your plans against the zoning code. If something doesn't comply, you get a Zoning Code Refusal Letter listing the problems.

You then have 45 days to file an appeal with the Zoning Board of Appeal through its online portal. The fee is $150, and you must attend a public hearing. An internal ADU that adds square footage or changes the exterior can also need relief. [3]

Grants and loans from the Boston Home Center

The ADU Financial Assistance Program helps income-eligible owner-occupants of 1-3 unit homes:

To qualify you must have under $100,000 in financial assets (with some exclusions) and be current on city and housing payments. You must also have attended both the ADU Design Workshop and the ADU Budget Workshop within 3 months before applying, and be working with a licensed architect. Call 617-635-4663 or email adu@boston.gov. See our ADU financing guide for other options. [2][4]

Renting out your ADU

No one can move in until ISD issues a Certificate of Occupancy after final inspections.

If you rent the ADU, you must register it with the city every year. The open period is January 2 to June 30, but you can do the first registration as soon as you have the certificate. You must register even if you charge no rent or your tenants are adult relatives not on the deed. Rental properties are inspected at least once every five years. [9]

Permit steps in Boston

  1. Confirm you're eligible and check your zoning

    You must own and live in a 1-, 2- or 3-family home. Look up your zoning district in the Zoning Viewer to see whether Mattapan's by-right rules apply or whether an attached or detached ADU would need ZBA relief. [1]

  2. Get feedback at a city ADU workshop

    Free virtual ADU Design and Budget Workshops run on the first Wednesday and Thursday of each month, 4-5:30 p.m., with staff from Housing, Planning and ISD. Both are required if you want city financial help. [4][10][11]

  3. Hire an architect and contractor

    You need plans stamped by a licensed architect or engineer and a certified plot plan. Massachusetts law generally requires a licensed general contractor to supervise construction. [2][8]

  4. Apply for a long-form building permit

    Apply through the ISD online portal or in person at 1010 Massachusetts Avenue. The fee is $50 plus $10 per $1,000 of estimated cost. [3][8]

  5. Go to the ZBA if you get a zoning refusal

    If the plans examiner issues a Zoning Code Refusal Letter, file an appeal with the Zoning Board of Appeal within 45 days ($150 fee) and attend the public hearing. [3]

  6. Build and pass inspections

    You need the printed permit card before work starts. The permit is valid for six months, and work must continue without interruption. [3]

  7. Get the Certificate of Occupancy and register the rental

    After final inspection, ISD issues a Certificate of Occupancy. If you rent the ADU, register it with the city each year. [9]

Who to call

Questions people ask

Do I have to live in my Boston home to build an ADU?

Yes. Boston requires you to be an owner-occupant of a 1-, 2- or 3-family home. The state law that banned owner-occupancy rules does not apply in Boston. [1][2]

Can I build a backyard cottage in Boston?

In Mattapan's R1 and R2 subdistricts, yes, without special approval if it meets the size and dimensional rules. Elsewhere the zoning code doesn't currently allow detached ADUs, so you may need a variance from the Zoning Board of Appeal. [1][2]

How long does an ADU permit take in Boston?

ISD review takes about 5 weeks for a complete application, longer if you need zoning relief. The city estimates up to 3 months for the permitting phase overall. [2][3]

How much does an ADU permit cost in Boston?

The long-form building permit is $50 plus $10 per $1,000 of estimated work cost. A zoning appeal adds $150, and trade permits have their own fees. [3][8]

Does the city help pay for ADUs?

Yes, for income-eligible owner-occupants. The Boston Home Center offers a grant of up to $7,500 for design and permitting costs and a 0% deferred loan of up to $50,000 for construction. [4]

Will an ADU raise my property taxes?

Yes. The city says an ADU will affect your property taxes once the property is reassessed, like any major home improvement. [2]

Sources (12)
  1. Research your ADU Options, City of Boston, Mayor's Office of Housing. Accessed October 9, 2026.
  2. Frequently Asked ADU Questions, City of Boston, Mayor's Office of Housing. Accessed October 9, 2026.
  3. Permitting Your Accessory Dwelling Unit, City of Boston, Mayor's Office of Housing. Accessed October 9, 2026.
  4. ADU Financial Assistance Program, City of Boston, Boston Home Center. Accessed October 9, 2026.
  5. 760 CMR 71.00: Protected Use Accessory Dwelling Units, Executive Office of Housing and Livable Communities. Accessed October 9, 2026.
  6. Article 60 Residential Zoning Amendment Fact Sheet, Boston Planning & Development Agency. Accessed October 9, 2026.
  7. Neighborhood Housing, City of Boston Planning Department. Accessed October 9, 2026.
  8. Long-Form Permit, City of Boston, Inspectional Services Department. Accessed October 9, 2026.
  9. Moving In: What's Next After Your ADU is Built?, City of Boston, Mayor's Office of Housing. Accessed October 9, 2026.
  10. Accessory Dwelling Units (ADUs) in Boston, City of Boston, Mayor's Office of Housing. Accessed October 9, 2026.
  11. Planning Your Accessory Dwelling Unit, City of Boston, Mayor's Office of Housing. Accessed October 9, 2026.
  12. Inspectional Services, City of Boston. Accessed October 9, 2026.

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